1. Field of the Invention
The present invention relates to a photoelectric conversion element, and more particularly to a photoelectric conversion element usable for an image processing apparatus such as a facsimile terminal equipment, a digital copying machine, or an image reader.
2. Related Background Art
Thin film semiconductors made of non-single crystal silicon (polysilicon, crystalline silicon and amorphous silicon) are suitably used for photoelectric conversion elements usable in larger or longer photoelectric conversion devices. The photoelectric conversion elements using thin film semiconductors are divided into two types of a primary photo current type (photodiode type) and a secondary photo current type. The primary photo current type is involved in a photoelectric conversion element in which the photoelectric conversion is made by picking up electrons and holes produced by the incident light, but has a problem that the photo current taken out as the output is small. On the other hand, a photoelectric conversion element of the secondary photo current type can produce a larger photo current (secondary photo current) than the photoelectric conversion element of the primary photo current type, and thus has many uses.
FIG. 1 is a schematic view illustrating the constitution of a photoelectric conversion element of the secondary photo current type. In FIG. 1, 1011 is an insulating substrate made of a glass, 1012 is a photoconductive semiconducting layer made of CdS.multidot.Se or amorphous silicon hydride (thereinafter abbreviated as "a-Si:H"), 1013a and 1013b are impurity layers for the ohmic contact, and 1014a and 1014b are electrodes. With such a constitution, if the voltage is applied across the electrodes 1014a, 1014b, a large secondary photo current flows when the light is incident from the side of the substrate 1011 or the electrodes 1014a, 1014b, so that the photoelectric conversion is made.
Further, photoelectric conversion elements of the thin film transistor type provided with auxiliary electrodes have been proposed for the stabilization and improvement of the characteristics (photo current or dark current). FIG. 2 is a schematic view illustrating the constitution of a photoelectric conversion element of the thin film transistor type provided with an auxiliary electrode. In FIGS. 1 and 2, like numerals refer to the like parts. 1015 is a transparent or opaque gate electrode, and 1016 is a gate insulating layer made of SiN.sub.x formed by the plasma CVD method, for example. Note that 1014a is a drain electrode, and 1014b is a source electrode, the voltages being applied to three electrodes 1014a, 1014b, 1015, respectively.
Examples of the gate voltage V.sub.G dependency of the photo current I.sub.p and the dark current I.sub.d, and the linearity r (I.sub.p .varies.F.sup.r) in the light quantity dependence, when the ray of light having the illuminance F is incident across the electrodes 1014a and 1014b of the photoelectric conversion element of the secondary photo current type, as shown in FIG. 2, are shown in FIGS. 3 and 4, respectively. Further, an example of the gate voltage V.sub.G dependency of the light response speed (T.sub.ON : rise time, T.sub.OFF : fall time) when the pulse light is incident is shown in FIG. 5.
The photoelectric conversion element of the thin film transistor type as shown in FIG. 1 is controlled for the stabilization of the characteristics so that an energy band gap of the photoconductive semiconductor layer may be substantially depleted. If the band gap of the photoconductive semiconductor layer is constant within the layer, electrons and holes produced within the photoconductive semiconductor layer are moved in opposite directions to each other owing to the slope of the band gap, with the irradiation of the light to the photoconductive semiconductor layer, whereby holes are collected in the vicinity of an interface between the photoconductive semiconductor layer and the insulating layer, and electrons are collected on a surface of the photoconductive semiconductor layer on the side of the first and second electrodes. Therefore, there is a problem that the photo current varies with the time as shown in FIG. 6, because it is subjected to the influence of traps due to interface defects and surface defects.
This point will be further described with reference to the drawings. In order to obtain a greater photo current, it is necessary that the gate voltage V.sub.G is a negative voltage as close to zero volt as possible. That is, it is necessary that the semiconductor layer 1012 is placed in a state depleted as weakly as possible so as not to cause the inversion of semiconductor. The reason will be described in the following.
This photoconductive type photo-sensor is operated in a state where a negative gate voltage (V.sub.G) is applied. At this time, the band of the photoconductive semiconductor 1012 is normally depleted about 1 .mu.m with a space charge distribution according to the Poisson's equation. That is, a portion of the photoconductive semiconductor layer 1012 on the side of the gate electrode 1015 is particularly strongly denser with the p-type. At this time, as a smaller voltage (larger absolute value) is applied to the gate electrode 1015, the linearity in the light quantity dependence is better, but the photo current decreases. This is believed due to the fact that as the photoconductive semiconductor layer 1012 is denser with the p-type, the lives of carriers (electrons in this case) are shorter, so that the gain G of the secondary photo current and thus the photo current decrease.
It should be noted that G is given by EQU G=.mu..tau.E/L
(.mu.: moving velocity of electron, .tau.: life of electron, E: electric field, L: electrode to electrode distance).
On the other hand, in order to increase the light response speed of the photoconductive type photo-sensor, it is necessary to have a smaller gate voltage (larger absolute value). This is believed due to the fact that there is a higher probability that the capture and emission of carriers are made in relatively shallow traps, as the gate voltage is smaller, so that the time constant of capture and emission is greater. In the following, this respect will be described in detail.
In an electron storage state, the light response speed is greatly influenced by the capture and emission process of electrons to a localized level, as shown in FIG. 7A. As shown in FIG. 7B, the rise time of output current is shorter with a larger V.sub.g, because it is determined by the capture process to a shallow level N.sub.A which is a main capture level in the electron storage state. On the other hand, the fall time is dominated by the electron emission process from N.sub.A and the capture (recombination) process to a recombination center N.sub.o which is a deep level, but as the emission of electrons is very fast, it is believed to be the recombination process that is rate-determining. That is, since excess electrons exist in the recombination process in the electron storage state, the capture of holes to the recombination center is rate-determining, but as the recombination center of holes is farther away from the valence band with a smaller V.sub.g (larger absolute value), the rise time is slower with the smaller V.sub.g.
On the other hand, in the hole storage state, the light response speed is greatly influenced by the capture and emission process to a localized level, as shown in FIG. 8A. That is, since the rise time of output current is determined by the capture process to N.sub.o which is a capture level of holes, as shown in FIG. 8B, it is shorter with a smaller V.sub.g. On the other hand, the fall time is determined by the emission process of holes from N.sub.o and the recombination process at N.sub.A, but since the emission of holes is much slower and thus rate-determining, it is faster with the smaller V.sub.g.
FIG. 5 is a graph representation illustrating the relation of T.sub.on and T.sub.off to V.sub.g. From the figure, the rise time T.sub.on has a maximum peak value near a certain value of V.sub.g (near a flat band voltage), in which in the negative electric field side thereof, the improvement (i) in the hole trap rate-determination is seen, while in the positive electric field side, the improvement (ii) in the electron rate-determination is seen. Also, the fall time T.sub.off increases monotonically with the increase of V.sub.g, as shown in the figure, with the improvement in the hole trap rate-determination (i) and the recombination rate-determination (ii). That is, in order to improve the light response characteristics, it is requisite that the storage state of holes is intensified by having a smaller V.sub.g (larger absolute value), and the shallow capture level is used for the holes. At the same time, it is necessary that the recombination of holes is increased in its spacial region to reduce the emission amount of holes.
As above described, it was difficult to make excellent both the photo current and the light response speed in the photoconductive type photo-sensor.